greybus: connection: add per-connection request handlers

Add a connection request-handler field to struct gb_connection that is
set when the connection is enabled.

This is a step towards removing the legacy protocol abstraction from
core, and will also be used to implement unidirectional connection
states (e.g. only outgoing operations are allowed).

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>